Coherent detection scanning systems used for ranging, doppler velocity or imaging applications are known in the prior art. A problem that arises in high performance coherent detection scanning systems is that of lag angle. The signal reflected by a target arrives at the optical antenna a finite amount of time after being transmitted from the optical antenna, the time delay being equal to 2 R/c, where R is the range of the target and c is the speed of light. Thus, the position of the scanning optical antenna system has moved appreciably during this time. The angular change of the scanning antenna is the lag angle. It causes the received signal to be misaligned with respect to the portion of the transmitting signal used as the local oscillator reference signal.
The output signal of the detector is found to be adversely affected by this misalignment, resulting in an increasing signal loss as the misalignment increases. This effectively limits the scanning rate and results in a degraded performance of the system.
Another problem encountered in high performance coherent detection scanning systems occurs when the spatially combined local oscillator and received signals are projected on different regions of a detector as the position of the scanning antenna changes. This results in fluctuations in the detector output signal due to the non-uniformity of the detector surface.